Transformers are used in many types of electronic device to perform such functions as transforming impedances, linking single-ended circuitry with balanced circuitry or vice versa and providing electrical isolation. However, not all transformers have all of these properties. For example, an autotransformer does not provide electrical isolation.
Transformers operating at audio and radio frequencies up to VHF are commonly built as coupled primary and secondary windings around a high permeability core. Current in the windings generates a magnetic flux. The core contains the magnetic flux and increases the coupling between the windings. A transformer operable in this frequency range can also be realized using an optical-coupler. An opto-coupler used in this mode is referred to in the art as an opto-isolator.
In transformers based on coupled windings or opto-couplers, the input electrical signal is converted to a different form (i.e., a magnetic flux or photons) that interacts with an appropriate transforming structure (i.e., another winding or a light detector), and is re-constituted as an electrical signal at the output. For example, an opto-coupler converts an input electrical signal to photons using a light-emitting diode. The photons pass through an optical fiber or free space that provides isolation. A photodiode illuminated by the photons generates an output electrical signal from the photon stream. The output electrical signal is a replica of the input electrical signal.
At UHF and microwave frequencies, coil-based transformers become impractical due to such factors as losses in the core, losses in the windings, capacitance between the windings, and a difficulty to make them small enough to prevent wavelength-related problems. Transformers for such frequencies are based on quarter-wavelength transmission lines, e.g., Marchand type, series input/parallel output connected lines, etc. Transformers also exist that are based on micro-machined coupled coils sets and are small enough that wavelength effects are unimportant. However such transformers have issues with high insertion loss.
All the transformers just described for use at UHF and microwave frequencies have dimensions that make them less desirable for use in modern miniature, high-density applications such as cellular telephones. Such transformers also tend to be high in cost because they are not capable of being manufactured by a batch process and because they are essentially an off-chip solution. Moreover, although such transformers typically have a bandwidth that is acceptable for use in cellular telephones, they typically have an insertion loss greater than 1 dB, which is too high.
Opto-couplers are not used at UHF and microwave frequencies due to the junction capacitance of the input LED, non-linearities inherent in the photodetector, limited power handling capability and insufficient isolation to give good common mode rejection.
Above-mentioned U.S. patent application Ser. No. 10/699,481, of which this disclosure is a continuation-in-part, discloses a film acoustically-coupled transformer (FACT) based on decoupled stacked bulk acoustic resonators (DSBARs). A DSBAR is composed of a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. FIG. 1A schematically illustrates an embodiment 100 of such FACT. FACT 100 has a first decoupled stacked bulk acoustic resonator (DSBAR) 106 and a second DSBAR 108 suspended above a cavity 104 in a substrate 102. DSBAR 106 has a lower FBAR 110, an upper FBAR 120 stacked on lower FBAR 110, and an acoustic coupler 130 between them, and DSBAR 108 has a lower FBAR 150, an upper FBAR 160 stacked on lower FBAR 150, and an acoustic coupler 170 between them. Each of the FBARs has opposed planar electrodes and a piezoelectric element between the electrodes. For example, FBAR 110 has opposed planar electrodes 112 and 114 with a piezoelectric element 116 between them.
FACT 100 additionally has a first electrical circuit 141 interconnecting the lower FBAR 110 of DSBAR 106 and the lower FBAR 150 of DSBAR 108 and a second electrical circuit 142 interconnecting the upper FBAR 120 of DSBAR 106 and the upper FBAR 160 of DSBAR 108.
In the embodiment of the above-described FACT shown in FIG. 1A, electrical circuit 141 connects lower FBARs 110 and 150 in anti-parallel and to terminals 143 and 144 and electrical circuit 142 connects upper FBARs 120 and 160 in series between terminals 145 and 146. In the example shown, electrical circuit 142 additionally has a center-tap terminal 147 connected to electrodes 122 and 162 of upper FBARs 120 and 160, respectively. This embodiment has a 1:4 impedance transformation ratio between electrical circuit 141 and electrical circuit 142 or a 4:1 impedance transformation ratio between electrical circuit 142 and electrical circuit 141.
In other embodiments, electrical circuit 141 electrically connects the lower FBARs 110 and 150 either in anti-parallel or in series, and electrical circuit 142 electrically connects the upper FBARs either in anti-parallel or in series.
All embodiments of the above-described FACT are small in size, are capable of linking single-ended circuitry with balanced circuitry or vice versa, and provide electrical isolation between primary and secondary. The embodiments specifically described above are also nominally electrically balanced.
The embodiment shown in FIG. 1A is of particular interest for a number of applications. However, although this embodiment is nominally electrically balanced, its common mode rejection is less than many potential applications require. The common-mode rejection of a differential device is quantified by a common-mode rejection ratio (CMRR), which is the ratio of the differential-mode voltage gain to the common-mode voltage gain of the differential device.
One approach to increasing the common-mode rejection ratio is to increase the thickness of the acoustic decoupler. However, increasing the thickness of the acoustic decoupler causes the frequency response of the FACT to exhibit spurious artifacts caused by the ability of the thicker acoustic decoupler to support more than a single acoustic mode. Such spurious response artifacts are undesirable in many applications.
What is needed, therefore, is an FACT that has the advantages of the FACT described above, but that has an increased common mode rejection ratio and a smooth frequency response.